Automatic type class derivation for Cats

kittens: automatic type class derivation for Cats and generic utility functions

kittens is a Scala library which provides instances of type classes from the Cats library for arbitrary algebraic data types using shapeless-based automatic type class derivation. It also provides some utility functions related to cats.Applicative such as lift, traverse and sequence to HList, Record and arbitrary parameter list.

kittens image

kittens is part of the Typelevel family of projects. It is an Open Source project under the Apache License v2, hosted on github. Binary artefacts will be published to the Sonatype OSS Repository Hosting service and synced to Maven Central.

It is current in a pre-alpha state, but should be ready for an initial release in the very near future.

Build Status Stories in Ready Gitter

Auto derived Examples

scala> import cats.derived._, functor._, legacy._
scala> import cats.Functor

scala> case class Cat[Food](food: Option[Food], foods: List[Food])
defined class Cat

scala> val f = Functor[Cat]
f: cats.Functor[Cat] = cats.derived.MkFunctor2$$anon$4@782b2ad1

scala> val cat = Cat(Some(1), List(2, 3))
cat: Cat[Int] = Cat(Some(1),List(2, 3))

scala> + 1)
res3: Cat[Int] = Cat(Some(2),List(3, 4))

Sequence examples

scala> import cats.implicits._, cats.sequence._
import cats.implicits._
import cats.sequence._

scala> val f1 = (_: String).length
f1: String => Int = <function1>

scala> val f2 = (_: String).reverse
f2: String => String = <function1>

scala> val f3 = (_: String).toFloat
f3: String => Double = <function1>

scala> val f = sequence(f1, f2, f3)
f: String => shapeless.::[Int,shapeless.::[String,shapeless.::[Float,shapeless.HNil]]] = <function1>

scala> f("42.0")
res0: shapeless.::[Int,shapeless.::[String,shapeless.::[Float,shapeless.HNil]]] = 4 :: 0.24 :: 42.0 :: HNil

//or generic over ADTs
scala>  case class MyCase(a: Int, b: String, c: Float)
defined class MyCase

scala>  val myGen = sequenceGeneric[MyCase]
myGen: cats.sequence.sequenceGen[MyCase] = cats.sequence.SequenceOps$sequenceGen@63ae3243

scala> val f = myGen(a = f1, b = f2, c = f3)
f: String => MyCase = <function1>

scala> f("42.0")
res1: MyCase = MyCase(4,0.24,42.0)

Traverse works similarly but you need a Poly.

Lift examples

scala> import cats._, implicits._, lift._
import cats._
import implicits._
import lift._

scala> def foo(x: Int, y: String, z: Float) = s"$x - $y - $z"

scala> val lifted = Applicative[Option].liftA(foo _)
lifted: (Option[Int], Option[String], Option[Float]) => Option[String] = <function3>

scala> lifted(Some(1), Some("a"), Some(3.2f))
res0: Option[String] = Some(1 - a - 3.2)

kittens and Typelevel Scala

Typelevel Scala provides a partial fix for SI-7046 which can present obstacles to the uses of shapeless's Generic and LabelledGeneric for the sealed trait at the root of an ADT such as you find in Kittens. If it appears that these two type classes are unable to find (all of) the subclasses of an ADT root trait then please try using Typelevel Scala and see if it resolves the issue.

To use Typelevel Scala you should,

  • Update your project/ to require SBT 0.13.13-M1 or later,

  • Add the following to your build.sbt immediately next to where you set scalaVersion,

    scalaOrganization := "org.typelevel"

If this does resolve the problem, please lend your support to the pull request being merged in Lightbend Scala.


The Kittens project supports the Typelevel code of conduct and wants all of its channels (mailing list, Gitter, github, etc.) to be welcoming environments for everyone.

Building kittens

kittens is built with SBT 0.13.9 or later, and its master branch is built with Scala 2.11.7 by default.